Method and apparatus for video coding

ABSTRACT

Aspects of the disclosure provide methods, apparatuses, and non-transitory computer-readable storage mediums for video encoding/decoding. In a method, prediction information of a current block of a coding unit tree in a coded bit stream is decoded. The prediction information indicates at least one allowed block partitioning structure for the current block. A sub-block transform (SBT) mode is determined for the current block based on the prediction information indicating that SBT is used for the current block. A partition of the current block based on the SBT mode is different from a partition of the current block based on the at least one allowed block partitioning structure. The current block is reconstructed based on the SBT mode.

INCORPORATION BY REFERENCE

This present application claims the benefit of priority to U.S.Provisional Application No. 62/891,839, “INTERACTION BETWEEN CUPARTITIONS AND SUB-BLOCK TRANSFORM” filed on Aug. 26, 2019, which isincorporated by reference herein in its entirety.

TECHNICAL FIELD

The present disclosure describes embodiments generally related to videocoding.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Video coding and decoding can be performed using inter-pictureprediction with motion compensation. Uncompressed digital video caninclude a series of pictures, each picture having a spatial dimensionof, for example, 1920×1080 luminance samples and associated chrominancesamples. The series of pictures can have a fixed or variable picturerate (informally also known as frame rate) of, for example, 60 picturesper second or 60 Hz. Uncompressed video has significant bitraterequirements. For example, 1080p60 4:2:0 video at 8 bit per sample(1920×1080 luminance sample resolution at 60 Hz frame rate) requiresclose to 1.5 Gbit/s bandwidth. An hour of such video requires more than600 GBytes of storage space.

One purpose of video coding and decoding can be the reduction ofredundancy in the input video signal, through compression. Compressioncan help reduce the aforementioned bandwidth or storage spacerequirements, in some cases by two orders of magnitude or more. Bothlossless and lossy compression, as well as a combination thereof can beemployed. Lossless compression refers to techniques where an exact copyof the original signal can be reconstructed from the compressed originalsignal. When using lossy compression, the reconstructed signal may notbe identical to the original signal, but the distortion between originaland reconstructed signals is small enough to make the reconstructedsignal useful for the intended application. In the case of video, lossycompression is widely employed. The amount of distortion tolerateddepends on the application; for example, users of certain consumerstreaming applications may tolerate higher distortion than users oftelevision distribution applications. The compression ratio achievablecan reflect that: higher allowable/tolerable distortion can yield highercompression ratios.

A video encoder and decoder can utilize techniques from several broadcategories, including, for example, motion compensation, transform,quantization, and entropy coding.

Video codec technologies can include techniques known as intra coding.In intra coding, sample values are represented without reference tosamples or other data from previously reconstructed reference pictures.In some video codecs, the picture is spatially subdivided into blocks ofsamples. When all blocks of samples are coded in intra mode, thatpicture can be an intra picture. Intra pictures and their derivationssuch as independent decoder refresh pictures, can be used to reset thedecoder state and can, therefore, be used as the first picture in acoded video bitstream and a video session, or as a still image. Thesamples of an intra block can be exposed to a transform, and thetransform coefficients can be quantized before entropy coding. Intraprediction can be a technique that minimizes sample values in thepre-transform domain. In some cases, the smaller the DC value after atransform is, and the smaller the AC coefficients are, the fewer thebits that are required at a given quantization step size to representthe block after entropy coding.

Traditional intra coding such as known from, for example MPEG-2generation coding technologies, does not use intra prediction. However,some newer video compression technologies include techniques thatattempt, from, for example, surrounding sample data and/or metadataobtained during the encoding/decoding of spatially neighboring, andpreceding in decoding order, blocks of data. Such techniques arehenceforth called “intra prediction” techniques. Note that in at leastsome cases, intra prediction is only using reference data from thecurrent picture under reconstruction and not from reference pictures.

There can be many different forms of intra prediction. When more thanone of such techniques can be used in a given video coding technology,the technique in use can be coded in an intra prediction mode. Incertain cases, modes can have submodes and/or parameters, and those canbe coded individually or included in the mode codeword. Which codewordto use for a given mode/submode/parameter combination can have an impactin the coding efficiency gain through intra prediction, and so can theentropy coding technology used to translate the codewords into abitstream.

A certain mode of intra prediction was introduced with H.264, refined inH.265, and further refined in newer coding technologies such as jointexploration model (JEM), versatile video coding (VVC), and benchmark set(BMS). A predictor block can be formed using neighboring sample valuesbelonging to already available samples. Sample values of neighboringsamples are copied into the predictor block according to a direction. Areference to the direction in use can be coded in the bitstream or maybe predicted itself.

Referring to FIG. 1A, depicted in the lower right is a subset of ninepredictor directions known from H.265's 33 possible predictor directions(corresponding to the 33 angular modes of the 35 intra modes). The pointwhere the arrows converge (101) represents the sample being predicted.The arrows represent the direction from which the sample is beingpredicted. For example, arrow (102) indicates that sample (101) ispredicted from a sample or samples to the upper right, at a 45 degreeangle from the horizontal. Similarly, arrow (103) indicates that sample(101) is predicted from a sample or samples to the lower left of sample(101), in a 22.5 degree angle from the horizontal.

Still referring to FIG. 1A, on the top left there is depicted a squareblock (104) of 4×4 samples (indicated by a dashed, boldface line). Thesquare block (104) includes 16 samples, each labelled with an “S”, itsposition in the Y dimension (e.g., row index) and its position in the Xdimension (e.g., column index). For example, sample S21 is the secondsample in the Y dimension (from the top) and the first (from the left)sample in the X dimension. Similarly, sample S44 is the fourth sample inblock (104) in both the Y and X dimensions. As the block is 4×4 samplesin size, S44 is at the bottom right. Further shown are reference samplesthat follow a similar numbering scheme. A reference sample is labelledwith an R, its Y position (e.g., row index) and X position (columnindex) relative to block (104). In both H.264 and H.265, predictionsamples neighbor the block under reconstruction; therefore no negativevalues need to be used.

Intra picture prediction can work by copying reference sample valuesfrom the neighboring samples as appropriated by the signaled predictiondirection. For example, assume the coded video bitstream includessignaling that, for this block, indicates a prediction directionconsistent with arrow (102). That is, samples are predicted from aprediction sample or samples to the upper right, at a 45 degree anglefrom the horizontal. In that case, samples S41, S32, S23, and S14 arepredicted from the same reference sample R05. Sample S44 is thenpredicted from reference sample R08.

In certain cases, the values of multiple reference samples may becombined, for example through interpolation, in order to calculate areference sample; especially when the directions are not evenlydivisible by 45 degrees.

The number of possible directions has increased as video codingtechnology has developed. In H.264 (year 2003), nine different directioncould be represented. That increased to 33 in H.265 (year 2013), andJEM/VVC/BMS, at the time of disclosure, can support up to 93 directions.Experiments have been conducted to identify the most likely directions,and certain techniques in the entropy coding are used to represent thoselikely directions in a small number of bits, accepting a certain penaltyfor less likely directions. Further, the directions themselves cansometimes be predicted from neighboring directions used in neighboring,already decoded, blocks.

FIG. 1B shows a schematic (105) that depicts 65 intra predictiondirections according to JEM to illustrate the increasing number ofprediction directions over time.

The mapping of intra prediction directions bits in the coded videobitstream that represent the direction can be different from videocoding technology to video coding technology; and can range, forexample, from simple direct mappings of prediction direction to intraprediction mode, to codewords, to complex adaptive schemes involvingmost probable modes, and similar techniques. In all cases, however,there can be certain directions that are statistically less likely tooccur in video content than certain other directions. As the goal ofvideo compression is the reduction of redundancy, those less likelydirections will, in a well working video coding technology, berepresented by a larger number of bits than more likely directions.

Motion compensation can be a lossy compression technique and can relateto techniques where a block of sample data from a previouslyreconstructed picture or part thereof (reference picture), after beingspatially shifted in a direction indicated by a motion vector (MVhenceforth), is used for the prediction of a newly reconstructed pictureor picture part. In some cases, the reference picture can be the same asthe picture currently under reconstruction. MVs can have two dimensionsX and Y, or three dimensions, the third being an indication of thereference picture in use (the latter, indirectly, can be a timedimension).

In some video compression techniques, an MV applicable to a certain areaof sample data can be predicted from other MVs, for example from thoserelated to another area of sample data spatially adjacent to the areaunder reconstruction, and preceding that MV in decoding order. Doing socan substantially reduce the amount of data required for coding the MV,thereby removing redundancy and increasing compression. MV predictioncan work effectively, for example, because when coding an input videosignal derived from a camera (known as natural video) there is astatistical likelihood that areas larger than the area to which a singleMV is applicable move in a similar direction and, therefore, can in somecases be predicted using a similar motion vector derived from MVs of aneighboring area. That results in the MV found for a given area to besimilar or the same as the MV predicted from the surrounding MVs, andthat in turn can be represented, after entropy coding, in a smallernumber of bits than what would be used if coding the MV directly. Insome cases, MV prediction can be an example of lossless compression of asignal (namely: the MVs) derived from the original signal (namely: thesample stream). In other cases, MV prediction itself can be lossy, forexample because of rounding errors when calculating a predictor fromseveral surrounding MVs.

Various MV prediction mechanisms are described in H.265/HEVC (ITU-T Rec.H.265, “High Efficiency Video Coding”, December 2016). Out of the manyMV prediction mechanisms that H.265 offers, described herein is atechnique henceforth referred to as “spatial merge.”

Referring to FIG. 1C, a current block (111) can include samples thathave been found by the encoder during the motion search process to bepredictable from a previous block of the same size that has beenspatially shifted. Instead of coding that MV directly, the MV can bederived from metadata associated with one or more reference pictures,for example from the most recent (in decoding order) reference picture,using the MV associated with either one of five surrounding samples,denoted A0, A1, and B0, B1, B2 (112 through 116, respectively). InH.265, the MV prediction can use predictors from the same referencepicture that the neighboring block is using.

SUMMARY

Aspects of the disclosure provide methods and apparatuses for videoencoding/decoding. In some examples, an apparatus for video decodingincludes processing circuitry.

According to aspects of the disclosure, there is provided a method forvideo decoding in a decoder. In the method, prediction information of acurrent block of a coding unit tree in a coded bit stream is decoded.The prediction information indicates at least one allowed blockpartitioning structure for the current block. A sub-block transform(SBT) mode is determined for the current block based on the predictioninformation indicating that SBT is used for the current block. Apartition of the current block based on the SBT mode is different from apartition of the current block based on the at least one allowed blockpartitioning structure. The current block is reconstructed based on theSBT mode.

In an embodiment, the SBT mode for the current block is determined when(i) one of a width and a height of the current block is greater than afirst threshold and (ii) a partitioning depth associated with thecurrent block is less than a maximum allowed partitioning depth of thecoding unit tree. In an example, the SBT mode is determined not to be avertical SBT mode when the width of the current block is greater thanthe first threshold. In an example, the SBT mode is determined not to bea horizontal SBT mode when the height of the current block is greaterthan the first threshold. In an example, the SBT mode is determined notto be a half SBT mode when (i) the first threshold is 8 and (ii) the atleast one allowed partitioning structure includes a binary treepartitioning structure. In an example, the SBT mode is determined not tobe a quarter SBT mode when (i) the first threshold is 16 and (ii) the atleast one allowed partitioning structure includes a triple treepartitioning structure.

In an embodiment, the SBT mode for the current block is determined when(i) the current block is a chroma block and (ii) sizes of a plurality ofsub-blocks of the current block are greater than a second threshold. Thecurrent block can be partitioned into the plurality of sub-blocks basedon the SBT mode.

In an embodiment, a flag indicating a direction of the SBT mode is notincluded in the prediction information.

In an embodiment, a flag indicating a size of the SBT mode is notincluded in the prediction information.

In an embodiment, a flag indicating whether SBT is used for the currentblock is not included in the prediction information.

In an embodiment, a context model for a flag is determined based on apartitioning depth associated with the current block. The flag indicateswhether SBT is used for the current block.

In an embodiment, a context model for a flag is determined based on atleast one of a direction of the SBT mode, the width of the currentblock, and the height of the current block. The flag indicates a size ofthe SBT mode.

In an embodiment, the prediction information indicates that the currentblock is a non-merge inter block.

Aspects of the disclosure provide an apparatus configured to perform anyone or a combination of the methods for video decoding. In anembodiment, the apparatus includes processing circuitry that decodesprediction information of a current block of a coding unit tree in acoded bit stream is decoded. The prediction information indicates atleast one allowed block partitioning structure for the current block.The processing circuitry determines a sub-block transform (SBT) mode forthe current block based on the prediction information indicating thatSBT is used for the current block. A partition of the current blockbased on the SBT mode is different from a partition of the current blockbased on the at least one allowed block partitioning structure. Theprocessing circuitry reconstructs the current block based on the SBTmode.

Aspects of the disclosure also provide a non-transitorycomputer-readable medium storing instructions which when executed by acomputer for video decoding cause the computer to perform any one or acombination of the methods for video decoding.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, the nature, and various advantages of the disclosedsubject matter will be more apparent from the following detaileddescription and the accompanying drawings in which:

FIG. 1A shows a schematic illustration of an exemplary subset of intraprediction modes;

FIG. 1B shows an illustration of exemplary intra prediction directions;

FIG. 1C shows a schematic illustration of a current block and itssurrounding spatial merge candidates in one example;

FIG. 2 shows a schematic illustration of a simplified block diagram of acommunication system in accordance with an embodiment;

FIG. 3 shows a schematic illustration of a simplified block diagram of acommunication system in accordance with an embodiment;

FIG. 4 shows a schematic illustration of a simplified block diagram of adecoder in accordance with an embodiment;

FIG. 5 shows a schematic illustration of a simplified block diagram ofan encoder in accordance with an embodiment;

FIG. 6 shows a block diagram of an encoder in accordance with anotherembodiment;

FIG. 7 shows a block diagram of a decoder in accordance with anotherembodiment;

FIGS. 8A and 8B illustrate an example of a block partition by using aquad-tree plus binary tree (QTBT) partitioning structure and thecorresponding QTBT structure in accordance with an embodiment;

FIGS. 9A and 9B show examples of vertical center-side ternary treepartitioning and horizontal center-side ternary tree partitioning,respectively, in accordance with some embodiments;

FIGS. 10A-10D show exemplary sub-block transform (SBT) modes inaccordance with some embodiments;

FIGS. 11-15 show exemplary syntax related to SBT methods according tosome embodiments of the disclosure;

FIG. 16 shows a flow chart outlining an exemplary process in accordancewith an embodiment; and

FIG. 17 shows a schematic illustration of a computer system inaccordance with an embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

The present disclosure includes embodiments directed to interactionbetween CU partition and sub-block transform (SBT). The embodimentsinclude methods, apparatuses, and non-transitory computer-readablestorage mediums for improving performance of CU partition and SBT. Inaddition, a block may refer to a prediction block, a coding block, or acoding unit.

I. Video Encoder and Decoder

FIG. 2 illustrates a simplified block diagram of a communication system(200) according to an embodiment of the present disclosure. Thecommunication system (200) includes a plurality of terminal devices thatcan communicate with each other, via, for example, a network (250). Forexample, the communication system (200) includes a first pair ofterminal devices (210) and (220) interconnected via the network (250).In the FIG. 2 example, the first pair of terminal devices (210) and(220) performs unidirectional transmission of data. For example, theterminal device (210) may code video data (e.g., a stream of videopictures that are captured by the terminal device (210)) fortransmission to the other terminal device (220) via the network (250).The encoded video data can be transmitted in the form of one or morecoded video bitstreams. The terminal device (220) may receive the codedvideo data from the network (250), decode the coded video data torecover the video pictures and display video pictures according to therecovered video data. Unidirectional data transmission may be common inmedia serving applications and the like.

In another example, the communication system (200) includes a secondpair of terminal devices (230) and (240) that performs bidirectionaltransmission of coded video data that may occur, for example, duringvideoconferencing. For bidirectional transmission of data, in anexample, each terminal device of the terminal devices (230) and (240)may code video data (e.g., a stream of video pictures that are capturedby the terminal device) for transmission to the other terminal device ofthe terminal devices (230) and (240) via the network (250). Eachterminal device of the terminal devices (230) and (240) also may receivethe coded video data transmitted by the other terminal device of theterminal devices (230) and (240), and may decode the coded video data torecover the video pictures and may display video pictures at anaccessible display device according to the recovered video data.

In the FIG. 2 example, the terminal devices (210), (220), (230) and(240) may be illustrated as servers, personal computers and smart phonesbut the principles of the present disclosure may be not so limited.Embodiments of the present disclosure find application with laptopcomputers, tablet computers, media players and/or dedicated videoconferencing equipment. The network (250) represents any number ofnetworks that convey coded video data among the terminal devices (210),(220), (230) and (240), including for example wireline (wired) and/orwireless communication networks. The communication network (250) mayexchange data in circuit-switched and/or packet-switched channels.Representative networks include telecommunications networks, local areanetworks, wide area networks and/or the Internet. For the purposes ofthe present discussion, the architecture and topology of the network(250) may be immaterial to the operation of the present disclosureunless explained herein below.

FIG. 3 illustrates, as an example for an application for the disclosedsubject matter, the placement of a video encoder and a video decoder ina streaming environment. The disclosed subject matter can be equallyapplicable to other video enabled applications, including, for example,video conferencing, digital TV, storing of compressed video on digitalmedia including CD, DVD, memory stick, and the like.

A streaming system may include a capture subsystem (313) that caninclude a video source (301), for example a digital camera, creating forexample a stream of video pictures (302) that are uncompressed. In anexample, the stream of video pictures (302) includes samples that aretaken by the digital camera. The stream of video pictures (302),depicted as a bold line to emphasize a high data volume when compared toencoded video data (304) (or coded video bitstreams), can be processedby an electronic device (320) that includes a video encoder (303)coupled to the video source (301). The video encoder (303) can includehardware, software, or a combination thereof to enable or implementaspects of the disclosed subject matter as described in more detailbelow. The encoded video data (304) (or encoded video bitstream (304)),depicted as a thin line to emphasize the lower data volume when comparedto the stream of video pictures (302), can be stored on a streamingserver (305) for future use. One or more streaming client subsystems,such as client subsystems (306) and (308) in FIG. 3 can access thestreaming server (305) to retrieve copies (307) and (309) of the encodedvideo data (304). A client subsystem (306) can include a video decoder(310), for example, in an electronic device (330). The video decoder(310) decodes the incoming copy (307) of the encoded video data andcreates an outgoing stream of video pictures (311) that can be renderedon a display (312) (e.g., display screen) or other rendering device (notdepicted). In some streaming systems, the encoded video data (304),(307), and (309) (e.g., video bitstreams) can be encoded according tocertain video coding/compression standards. Examples of those standardsinclude ITU-T Recommendation H.265. In an example, a video codingstandard under development is informally known as Versatile Video Coding(VVC). The disclosed subject matter may be used in the context of VVC.

It is noted that the electronic devices (320) and (330) can includeother components (not shown). For example, the electronic device (320)can include a video decoder (not shown) and the electronic device (330)can include a video encoder (not shown) as well.

FIG. 4 shows a block diagram of a video decoder (410) according to anembodiment of the present disclosure. The video decoder (410) can beincluded in an electronic device (430). The electronic device (430) caninclude a receiver (431) (e.g., receiving circuitry). The video decoder(410) can be used in the place of the video decoder (310) in the FIG. 3example.

The receiver (431) may receive one or more coded video sequences to bedecoded by the video decoder (410); in the same or another embodiment,one coded video sequence at a time, where the decoding of each codedvideo sequence is independent from other coded video sequences. Thecoded video sequence may be received from a channel (401), which may bea hardware/software link to a storage device which stores the encodedvideo data. The receiver (431) may receive the encoded video data withother data, for example, coded audio data and/or ancillary data streams,that may be forwarded to their respective using entities (not depicted).The receiver (431) may separate the coded video sequence from the otherdata. To combat network jitter, a buffer memory (415) may be coupled inbetween the receiver (431) and an entropy decoder/parser (420) (“parser(420)” henceforth). In certain applications, the buffer memory (415) ispart of the video decoder (410). In others, it can be outside of thevideo decoder (410) (not depicted). In still others, there can be abuffer memory (not depicted) outside of the video decoder (410), forexample to combat network jitter, and in addition another buffer memory(415) inside the video decoder (410), for example to handle playouttiming. When the receiver (431) is receiving data from a store/forwarddevice of sufficient bandwidth and controllability, or from anisosynchronous network, the buffer memory (415) may not be needed, orcan be small. For use on best effort packet networks such as theInternet, the buffer memory (415) may be required, can be comparativelylarge and can be advantageously of adaptive size, and may at leastpartially be implemented in an operating system or similar elements (notdepicted) outside of the video decoder (410).

The video decoder (410) may include the parser (420) to reconstructsymbols (421) from the coded video sequence. Categories of those symbolsinclude information used to manage operation of the video decoder (410),and potentially information to control a rendering device such as arender device (412) (e.g., a display screen) that is not an integralpart of the electronic device (430) but can be coupled to the electronicdevice (430), as was shown in FIG. 4. The control information for therendering device(s) may be in the form of Supplemental EnhancementInformation (SEI messages) or Video Usability Information (VUI)parameter set fragments (not depicted). The parser (420) mayparse/entropy-decode the coded video sequence that is received. Thecoding of the coded video sequence can be in accordance with a videocoding technology or standard, and can follow various principles,including variable length coding, Huffman coding, arithmetic coding withor without context sensitivity, and so forth. The parser (420) mayextract from the coded video sequence, a set of subgroup parameters forat least one of the subgroups of pixels in the video decoder, based uponat least one parameter corresponding to the group. Subgroups can includeGroups of Pictures (GOPs), pictures, tiles, slices, macroblocks, CodingUnits (CUs), blocks, Transform Units (TUs), Prediction Units (PUs) andso forth. The parser (420) may also extract from the coded videosequence information such as transform coefficients, quantizer parametervalues, motion vectors, and so forth.

The parser (420) may perform an entropy decoding/parsing operation onthe video sequence received from the buffer memory (415), so as tocreate symbols (421).

Reconstruction of the symbols (421) can involve multiple different unitsdepending on the type of the coded video picture or parts thereof (suchas: inter and intra picture, inter and intra block), and other factors.Which units are involved, and how, can be controlled by the subgroupcontrol information that was parsed from the coded video sequence by theparser (420). The flow of such subgroup control information between theparser (420) and the multiple units below is not depicted for clarity.

Beyond the functional blocks already mentioned, the video decoder (410)can be conceptually subdivided into a number of functional units asdescribed below. In a practical implementation operating undercommercial constraints, many of these units interact closely with eachother and can, at least partly, be integrated into each other. However,for the purpose of describing the disclosed subject matter, theconceptual subdivision into the functional units below is appropriate.

A first unit is the scaler/inverse transform unit (451). Thescaler/inverse transform unit (451) receives a quantized transformcoefficient as well as control information, including which transform touse, block size, quantization factor, quantization scaling matrices,etc. as symbol(s) (421) from the parser (420). The scaler/inversetransform unit (451) can output blocks comprising sample values that canbe input into aggregator (455).

In some cases, the output samples of the scaler/inverse transform (451)can pertain to an intra coded block; that is: a block that is not usingpredictive information from previously reconstructed pictures, but canuse predictive information from previously reconstructed parts of thecurrent picture. Such predictive information can be provided by an intrapicture prediction unit (452). In some cases, the intra pictureprediction unit (452) generates a block of the same size and shape ofthe block under reconstruction, using surrounding already reconstructedinformation fetched from the current picture buffer (458). The currentpicture buffer (458) buffers, for example, partly reconstructed currentpicture and/or fully reconstructed current picture. The aggregator(455), in some cases, adds, on a per sample basis, the predictioninformation that the intra prediction unit (452) has generated to theoutput sample information as provided by the scaler/inverse transformunit (451).

In other cases, the output samples of the scaler/inverse transform unit(451) can pertain to an inter coded, and potentially motion compensatedblock. In such a case, a motion compensation prediction unit (453) canaccess reference picture memory (457) to fetch samples used forprediction. After motion compensating the fetched samples in accordancewith the symbols (421) pertaining to the block, these samples can beadded by the aggregator (455) to the output of the scaler/inversetransform unit (451) (in this case called the residual samples orresidual signal) so as to generate output sample information. Theaddresses within the reference picture memory (457) from where themotion compensation prediction unit (453) fetches prediction samples canbe controlled by motion vectors, available to the motion compensationprediction unit (453) in the form of symbols (421) that can have, forexample X, Y, and reference picture components. Motion compensation alsocan include interpolation of sample values as fetched from the referencepicture memory (457) when sub-sample exact motion vectors are in use,motion vector prediction mechanisms, and so forth.

The output samples of the aggregator (455) can be subject to variousloop filtering techniques in the loop filter unit (456). Videocompression technologies can include in-loop filter technologies thatare controlled by parameters included in the coded video sequence (alsoreferred to as coded video bitstream) and made available to the loopfilter unit (456) as symbols (421) from the parser (420), but can alsobe responsive to meta-information obtained during the decoding ofprevious (in decoding order) parts of the coded picture or coded videosequence, as well as responsive to previously reconstructed andloop-filtered sample values.

The output of the loop filter unit (456) can be a sample stream that canbe output to the render device (412) as well as stored in the referencepicture memory (457) for use in future inter-picture prediction.

Certain coded pictures, once fully reconstructed, can be used asreference pictures for future prediction. For example, once a codedpicture corresponding to a current picture is fully reconstructed andthe coded picture has been identified as a reference picture (by, forexample, the parser (420)), the current picture buffer (458) can becomea part of the reference picture memory (457), and a fresh currentpicture buffer can be reallocated before commencing the reconstructionof the following coded picture.

The video decoder (410) may perform decoding operations according to apredetermined video compression technology in a standard, such as ITU-TRec. H.265. The coded video sequence may conform to a syntax specifiedby the video compression technology or standard being used, in the sensethat the coded video sequence adheres to both the syntax of the videocompression technology or standard and the profiles as documented in thevideo compression technology or standard. Specifically, a profile canselect certain tools as the only tools available for use under thatprofile from all the tools available in the video compression technologyor standard. Also necessary for compliance can be that the complexity ofthe coded video sequence is within bounds as defined by the level of thevideo compression technology or standard. In some cases, levels restrictthe maximum picture size, maximum frame rate, maximum reconstructionsample rate (measured in, for example megasamples per second), maximumreference picture size, and so on. Limits set by levels can, in somecases, be further restricted through Hypothetical Reference Decoder(HRD) specifications and metadata for HRD buffer management signaled inthe coded video sequence.

In an embodiment, the receiver (431) may receive additional (redundant)data with the encoded video. The additional data may be included as partof the coded video sequence(s). The additional data may be used by thevideo decoder (410) to properly decode the data and/or to moreaccurately reconstruct the original video data. Additional data can bein the form of, for example, temporal, spatial, or signal noise ratio(SNR) enhancement layers, redundant slices, redundant pictures, forwarderror correction codes, and so on.

FIG. 5 shows a block diagram of a video encoder (503) according to anembodiment of the present disclosure. The video encoder (503) isincluded in an electronic device (520). The electronic device (520)includes a transmitter (540) (e.g., transmitting circuitry). The videoencoder (503) can be used in the place of the video encoder (303) in theFIG. 3 example.

The video encoder (503) may receive video samples from a video source(501) (that is not part of the electronic device (520) in the FIG. 5example) that may capture video image(s) to be coded by the videoencoder (503). In another example, the video source (501) is a part ofthe electronic device (520).

The video source (501) may provide the source video sequence to be codedby the video encoder (503) in the form of a digital video sample streamthat can be of any suitable bit depth (for example: 8 bit, 10 bit, 12bit, . . . ), any colorspace (for example, BT.601 Y CrCB, RGB, . . . ),and any suitable sampling structure (for example Y CrCb 4:2:0, Y CrCb4:4:4). In a media serving system, the video source (501) may be astorage device storing previously prepared video. In a videoconferencingsystem, the video source (501) may be a camera that captures local imageinformation as a video sequence. Video data may be provided as aplurality of individual pictures that impart motion when viewed insequence. The pictures themselves may be organized as a spatial array ofpixels, wherein each pixel can comprise one or more samples depending onthe sampling structure, color space, etc. in use. A person skilled inthe art can readily understand the relationship between pixels andsamples. The description below focuses on samples.

According to an embodiment, the video encoder (503) may code andcompress the pictures of the source video sequence into a coded videosequence (543) in real time or under any other time constraints asrequired by the application. Enforcing appropriate coding speed is onefunction of a controller (550). In some embodiments, the controller(550) controls other functional units as described below and isfunctionally coupled to the other functional units. The coupling is notdepicted for clarity. Parameters set by the controller (550) can includerate control related parameters (picture skip, quantizer, lambda valueof rate-distortion optimization techniques, . . . ), picture size, groupof pictures (GOP) layout, maximum motion vector allowed reference area,and so forth. The controller (550) can be configured to have othersuitable functions that pertain to the video encoder (503) optimized fora certain system design.

In some embodiments, the video encoder (503) is configured to operate ina coding loop. As an oversimplified description, in an example, thecoding loop can include a source coder (530) (e.g., responsible forcreating symbols, such as a symbol stream, based on an input picture tobe coded, and a reference picture(s)), and a (local) decoder (533)embedded in the video encoder (503). The decoder (533) reconstructs thesymbols to create the sample data in a similar manner as a (remote)decoder also would create (as any compression between symbols and codedvideo bitstream is lossless in the video compression technologiesconsidered in the disclosed subject matter). The reconstructed samplestream (sample data) is input to the reference picture memory (534). Asthe decoding of a symbol stream leads to bit-exact results independentof decoder location (local or remote), the content in the referencepicture memory (534) is also bit exact between the local encoder andremote encoder. In other words, the prediction part of an encoder “sees”as reference picture samples exactly the same sample values as a decoderwould “see” when using prediction during decoding. This fundamentalprinciple of reference picture synchronicity (and resulting drift, ifsynchronicity cannot be maintained, for example because of channelerrors) is used in some related arts as well.

The operation of the “local” decoder (533) can be the same as of a“remote” decoder, such as the video decoder (410), which has alreadybeen described in detail above in conjunction with FIG. 4. Brieflyreferring also to FIG. 4, however, as symbols are available andencoding/decoding of symbols to a coded video sequence by an entropycoder (545) and the parser (420) can be lossless, the entropy decodingparts of the video decoder (410), including the buffer memory (415) andthe parser (420) may not be fully implemented in the local decoder(533).

An observation that can be made at this point is that any decodertechnology except the parsing/entropy decoding that is present in adecoder also necessarily needs to be present, in substantially identicalfunctional form, in a corresponding encoder. For this reason, thedisclosed subject matter focuses on decoder operation. The descriptionof encoder technologies can be abbreviated as they are the inverse ofthe comprehensively described decoder technologies. Only in certainareas a more detail description is required and provided below.

During operation, in some examples, the source coder (530) may performmotion compensated predictive coding, which codes an input picturepredictively with reference to one or more previously coded picture fromthe video sequence that were designated as “reference pictures.” In thismanner, the coding engine (532) codes differences between pixel blocksof an input picture and pixel blocks of reference picture(s) that may beselected as prediction reference(s) to the input picture.

The local video decoder (533) may decode coded video data of picturesthat may be designated as reference pictures, based on symbols createdby the source coder (530). Operations of the coding engine (532) mayadvantageously be lossy processes. When the coded video data may bedecoded at a video decoder (not shown in FIG. 5), the reconstructedvideo sequence typically may be a replica of the source video sequencewith some errors. The local video decoder (533) replicates decodingprocesses that may be performed by the video decoder on referencepictures and may cause reconstructed reference pictures to be stored inthe reference picture cache (534). In this manner, the video encoder(503) may store copies of reconstructed reference pictures locally thathave common content as the reconstructed reference pictures that will beobtained by a far-end video decoder (absent transmission errors).

The predictor (535) may perform prediction searches for the codingengine (532). That is, for a new picture to be coded, the predictor(535) may search the reference picture memory (534) for sample data (ascandidate reference pixel blocks) or certain metadata such as referencepicture motion vectors, block shapes, and so on, that may serve as anappropriate prediction reference for the new pictures. The predictor(535) may operate on a sample block-by-pixel block basis to findappropriate prediction references. In some cases, as determined bysearch results obtained by the predictor (535), an input picture mayhave prediction references drawn from multiple reference pictures storedin the reference picture memory (534).

The controller (550) may manage coding operations of the source coder(530), including, for example, setting of parameters and subgroupparameters used for encoding the video data.

Output of all aforementioned functional units may be subjected toentropy coding in the entropy coder (545). The entropy coder (545)translates the symbols as generated by the various functional units intoa coded video sequence, by lossless compressing the symbols according totechnologies such as Huffman coding, variable length coding, arithmeticcoding, and so forth.

The transmitter (540) may buffer the coded video sequence(s) as createdby the entropy coder (545) to prepare for transmission via acommunication channel (560), which may be a hardware/software link to astorage device which would store the encoded video data. The transmitter(540) may merge coded video data from the video coder (503) with otherdata to be transmitted, for example, coded audio data and/or ancillarydata streams (sources not shown).

The controller (550) may manage operation of the video encoder (503).During coding, the controller (550) may assign to each coded picture acertain coded picture type, which may affect the coding techniques thatmay be applied to the respective picture. For example, pictures oftenmay be assigned as one of the following picture types:

An Intra Picture (I picture) may be one that may be coded and decodedwithout using any other picture in the sequence as a source ofprediction. Some video codecs allow for different types of intrapictures, including, for example Independent Decoder Refresh (“IDR”)Pictures. A person skilled in the art is aware of those variants of Ipictures and their respective applications and features.

A predictive picture (P picture) may be one that may be coded anddecoded using intra prediction or inter prediction using at most onemotion vector and reference index to predict the sample values of eachblock.

A bi-directionally predictive picture (B Picture) may be one that may becoded and decoded using intra prediction or inter prediction using atmost two motion vectors and reference indices to predict the samplevalues of each block. Similarly, multiple-predictive pictures can usemore than two reference pictures and associated metadata for thereconstruction of a single block.

Source pictures commonly may be subdivided spatially into a plurality ofsample blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16 sampleseach) and coded on a block-by-block basis. Blocks may be codedpredictively with reference to other (already coded) blocks asdetermined by the coding assignment applied to the blocks' respectivepictures. For example, blocks of I pictures may be codednon-predictively or they may be coded predictively with reference toalready coded blocks of the same picture (spatial prediction or intraprediction). Pixel blocks of P pictures may be coded predictively, viaspatial prediction or via temporal prediction with reference to onepreviously coded reference picture. Blocks of B pictures may be codedpredictively, via spatial prediction or via temporal prediction withreference to one or two previously coded reference pictures.

The video encoder (503) may perform coding operations according to apredetermined video coding technology or standard, such as ITU-T Rec.H.265. In its operation, the video encoder (503) may perform variouscompression operations, including predictive coding operations thatexploit temporal and spatial redundancies in the input video sequence.The coded video data, therefore, may conform to a syntax specified bythe video coding technology or standard being used.

In an embodiment, the transmitter (540) may transmit additional datawith the encoded video. The source coder (530) may include such data aspart of the coded video sequence. Additional data may comprisetemporal/spatial/SNR enhancement layers, other forms of redundant datasuch as redundant pictures and slices, SEI messages, VUI parameter setfragments, and so on.

A video may be captured as a plurality of source pictures (videopictures) in a temporal sequence. Intra-picture prediction (oftenabbreviated to intra prediction) makes use of spatial correlation in agiven picture, and inter-picture prediction makes uses of the (temporalor other) correlation between the pictures. In an example, a specificpicture under encoding/decoding, which is referred to as a currentpicture, is partitioned into blocks. When a block in the current pictureis similar to a reference block in a previously coded and still bufferedreference picture in the video, the block in the current picture can becoded by a vector that is referred to as a motion vector. The motionvector points to the reference block in the reference picture, and canhave a third dimension identifying the reference picture, in casemultiple reference pictures are in use.

In some embodiments, a bi-prediction technique can be used in theinter-picture prediction. According to the bi-prediction technique, tworeference pictures, such as a first reference picture and a secondreference picture that are both prior in decoding order to the currentpicture in the video (but may be in the past and future, respectively,in display order) are used. A block in the current picture can be codedby a first motion vector that points to a first reference block in thefirst reference picture, and a second motion vector that points to asecond reference block in the second reference picture. The block can bepredicted by a combination of the first reference block and the secondreference block.

Further, a merge mode technique can be used in the inter-pictureprediction to improve coding efficiency.

According to some embodiments of the disclosure, predictions, such asinter-picture predictions and intra-picture predictions are performed inthe unit of blocks. For example, according to the HEVC standard, apicture in a sequence of video pictures is partitioned into coding treeunits (CTU) for compression, the CTUs in a picture have the same size,such as 64×64 pixels, 32×32 pixels, or 16×16 pixels. In general, a CTUincludes three coding tree blocks (CTBs), which are one luma CTB and twochroma CTBs. Each CTU can be recursively quad-tree split into one ormultiple CUs. For example, a CTU of 64×64 pixels can be split into oneCU of 64×64 pixels, or 4 CUs of 32×32 pixels, or 16 CUs of 16×16 pixels.In an example, each CU is analyzed to determine a prediction type forthe CU, such as an inter prediction type or an intra prediction type.The CU is split into one or more prediction units (PUs) depending on thetemporal and/or spatial predictability. Generally, each PU includes aluma prediction block (PB), and two chroma PBs. In an embodiment, aprediction operation in coding (encoding/decoding) is performed in theunit of a prediction block. Using a luma prediction block as an exampleof a prediction block, the prediction block includes a matrix of values(e.g., luma values) for pixels, such as 8×8 pixels, 16×16 pixels, 8×16pixels, 16×8 pixels, and the like.

FIG. 6 shows a diagram of a video encoder (603) according to anotherembodiment of the disclosure. The video encoder (603) is configured toreceive a processing block (e.g., a prediction block) of sample valueswithin a current video picture in a sequence of video pictures, andencode the processing block into a coded picture that is part of a codedvideo sequence. In an example, the video encoder (603) is used in theplace of the video encoder (303) in the FIG. 3 example.

In an HEVC example, the video encoder (603) receives a matrix of samplevalues for a processing block, such as a prediction block of 8×8samples, and the like. The video encoder (603) determines whether theprocessing block is best coded using intra mode, inter mode, orbi-prediction mode using, for example, rate-distortion optimization.When the processing block is to be coded in intra mode, the videoencoder (603) may use an intra prediction technique to encode theprocessing block into the coded picture; and when the processing blockis to be coded in inter mode or bi-prediction mode, the video encoder(603) may use an inter prediction or bi-prediction technique,respectively, to encode the processing block into the coded picture. Incertain video coding technologies, merge mode can be an inter pictureprediction submode where the motion vector is derived from one or moremotion vector predictors without the benefit of a coded motion vectorcomponent outside the predictors. In certain other video codingtechnologies, a motion vector component applicable to the subject blockmay be present. In an example, the video encoder (603) includes othercomponents, such as a mode decision module (not shown) to determine themode of the processing blocks.

In the FIG. 6 example, the video encoder (603) includes the interencoder (630), an intra encoder (622), a residue calculator (623), aswitch (626), a residue encoder (624), a general controller (621), andan entropy encoder (625) coupled together as shown in FIG. 6.

The inter encoder (630) is configured to receive the samples of thecurrent block (e.g., a processing block), compare the block to one ormore reference blocks in reference pictures (e.g., blocks in previouspictures and later pictures), generate inter prediction information(e.g., description of redundant information according to inter encodingtechnique, motion vectors, merge mode information), and calculate interprediction results (e.g., predicted block) based on the inter predictioninformation using any suitable technique. In some examples, thereference pictures are decoded reference pictures that are decoded basedon the encoded video information.

The intra encoder (622) is configured to receive the samples of thecurrent block (e.g., a processing block), in some cases compare theblock to blocks already coded in the same picture, generate quantizedcoefficients after transform, and in some cases also intra predictioninformation (e.g., an intra prediction direction information accordingto one or more intra encoding techniques). In an example, the intraencoder (622) also calculates intra prediction results (e.g., predictedblock) based on the intra prediction information and reference blocks inthe same picture.

The general controller (621) is configured to determine general controldata and control other components of the video encoder (603) based onthe general control data. In an example, the general controller (621)determines the mode of the block, and provides a control signal to theswitch (626) based on the mode. For example, when the mode is the intramode, the general controller (621) controls the switch (626) to selectthe intra mode result for use by the residue calculator (623), andcontrols the entropy encoder (625) to select the intra predictioninformation and include the intra prediction information in thebitstream; and when the mode is the inter mode, the general controller(621) controls the switch (626) to select the inter prediction resultfor use by the residue calculator (623), and controls the entropyencoder (625) to select the inter prediction information and include theinter prediction information in the bitstream.

The residue calculator (623) is configured to calculate a difference(residue data) between the received block and prediction resultsselected from the intra encoder (622) or the inter encoder (630). Theresidue encoder (624) is configured to operate based on the residue datato encode the residue data to generate the transform coefficients. In anexample, the residue encoder (624) is configured to convert the residuedata from a spatial domain to a frequency domain, and generate thetransform coefficients. The transform coefficients are then subject toquantization processing to obtain quantized transform coefficients. Invarious embodiments, the video encoder (603) also includes a residuedecoder (628). The residue decoder (628) is configured to performinverse-transform, and generate the decoded residue data. The decodedresidue data can be suitably used by the intra encoder (622) and theinter encoder (630). For example, the inter encoder (630) can generatedecoded blocks based on the decoded residue data and inter predictioninformation, and the intra encoder (622) can generate decoded blocksbased on the decoded residue data and the intra prediction information.The decoded blocks are suitably processed to generate decoded picturesand the decoded pictures can be buffered in a memory circuit (not shown)and used as reference pictures in some examples.

The entropy encoder (625) is configured to format the bitstream toinclude the encoded block. The entropy encoder (625) is configured toinclude various information according to a suitable standard, such asthe HEVC standard. In an example, the entropy encoder (625) isconfigured to include the general control data, the selected predictioninformation (e.g., intra prediction information or inter predictioninformation), the residue information, and other suitable information inthe bitstream. Note that, according to the disclosed subject matter,when coding a block in the merge submode of either inter mode orbi-prediction mode, there is no residue information.

FIG. 7 shows a diagram of a video decoder (710) according to anotherembodiment of the disclosure. The video decoder (710) is configured toreceive coded pictures that are part of a coded video sequence, anddecode the coded pictures to generate reconstructed pictures. In anexample, the video decoder (710) is used in the place of the videodecoder (310) in the FIG. 3 example.

In the FIG. 7 example, the video decoder (710) includes an entropydecoder (771), an inter decoder (780), a residue decoder (773), areconstruction module (774), and an intra decoder (772) coupled togetheras shown in FIG. 7.

The entropy decoder (771) can be configured to reconstruct, from thecoded picture, certain symbols that represent the syntax elements ofwhich the coded picture is made up. Such symbols can include, forexample, the mode in which a block is coded (such as, for example, intramode, inter mode, bi-predicted mode, the latter two in merge submode oranother submode), prediction information (such as, for example, intraprediction information or inter prediction information) that canidentify certain sample or metadata that is used for prediction by theintra decoder (772) or the inter decoder (780), respectively, residualinformation in the form of, for example, quantized transformcoefficients, and the like. In an example, when the prediction mode isinter or bi-predicted mode, the inter prediction information is providedto the inter decoder (780); and when the prediction type is the intraprediction type, the intra prediction information is provided to theintra decoder (772). The residual information can be subject to inversequantization and is provided to the residue decoder (773).

The inter decoder (780) is configured to receive the inter predictioninformation, and generate inter prediction results based on the interprediction information.

The intra decoder (772) is configured to receive the intra predictioninformation, and generate prediction results based on the intraprediction information.

The residue decoder (773) is configured to perform inverse quantizationto extract de-quantized transform coefficients, and process thede-quantized transform coefficients to convert the residual from thefrequency domain to the spatial domain. The residue decoder (773) mayalso require certain control information (to include the QuantizerParameter (QP)), and that information may be provided by the entropydecoder (771) (data path not depicted as this may be low volume controlinformation only).

The reconstruction module (774) is configured to combine, in the spatialdomain, the residual as output by the residue decoder (773) and theprediction results (as output by the inter or intra prediction modulesas the case may be) to form a reconstructed block, that may be part ofthe reconstructed picture, which in turn may be part of thereconstructed video. It is noted that other suitable operations, such asa deblocking operation and the like, can be performed to improve thevisual quality.

It is noted that the video encoders (303), (503), and (603), and thevideo decoders (310), (410), and (710) can be implemented using anysuitable technique. In an embodiment, the video encoders (303), (503),and (603), and the video decoders (310), (410), and (710) can beimplemented using one or more integrated circuits. In anotherembodiment, the video encoders (303), (503), and (603), and the videodecoders (310), (410), and (710) can be implemented using one or moreprocessors that execute software instructions.

II. HEVC Block Partition Structure

A CTU, such as in HEVC, can be split into CUs by using a quad-treestructure denoted as a coding tree to adapt to various localcharacteristics. The decision on whether to code a picture area usinginter-picture (temporal) or intra-picture (spatial) prediction can bemade at the CU level. Each CU can be further split into one, two, orfour PUs according to a PU splitting type. Inside one PU, the sameprediction process is applied and the relevant information istransmitted to a decoder on a PU basis. After obtaining a residual blockby applying the prediction process based on the PU splitting type, a CUcan be partitioned into TUs according to another QT structure like thecoding tree for the CU.

One feature of the HEVC structure is that it has multiple partitionconceptions including CU, PU, and TU. In HEVC, a CU or a TU can belimited to a square shape, while a PU may be a square or rectangularshape for an inter predicted block. Rectangular shape PUs for intraprediction and transform that were proposed can be extended to be usedin joint exploration model (JEM).

III. Block Partitioning Structure Using QTBT

A quad-tree (QT) plus binary-tree (BT) plus ternary tree (TT)partitioning structure can be applied, such as in VVC test model (VTM).The quad-tree plus binary tree (QTBT) structure removes the concepts ofmultiple partition types (i.e., it removes the separation of the CU, PU,and TU concepts), and supports more flexibility for CU partition shapes.

In the QTBT structure, a CU can have either a square or rectangularshape. As shown in FIGS. 8A and 8B, a CTU is first partitioned by a QTstructure. The QT leaf nodes can be further partitioned by a BTstructure. There are two splitting types, symmetric horizontal splittingand symmetric vertical splitting, in the BT splitting. The BT leaf nodesare CUs, and segmentation into two CUs is used for prediction andtransform processing without any further partitioning. Accordingly, aCU, PU, and TU can have the same block size in the QTBT structure.

A CU sometimes can include CBs of different color components, such as inJEM. For example, one CU can contain one luma CB and two chroma CBs inthe case of P and B slices with the 4:2:0 chroma format. In otherexamples, a CU can include CBs of a single component, e.g., one CU cancontain only one luma CB or just two chroma CBs in the case of I slices.

The following parameters are defined for the QTBT partitioning scheme:

-   -   CTU size: the root node size of a QT, for example as in HEVC    -   MinQTSize: the minimum allowed QT leaf node size    -   MaxBTSize: the maximum allowed BT root node size    -   MaxBTDepth: the maximum allowed BT depth    -   MinBTSize: the minimum allowed BT leaf node size

In one example of the QTBT partitioning structure, the CTU size is setas 128×128 luma samples with two corresponding 64×64 blocks of chromasamples, the MinQTSize is set as 16×16, the MaxBTSize is set as 64×64,the MinBTSize (for both width and height) is set as 4×4, and theMaxBTDepth is set as 4. The QT partitioning is applied to the CTU firstto generate QT leaf nodes. The QT leaf nodes may have a size from 16×16(i.e., the MinQTSize) to 128×128 (i.e., the CTU size). If the leaf QTnode is 128×128, it will not be further split by the BT since the sizeexceeds the MaxBTSize (i.e., 64×64). Otherwise, the leaf QT node couldbe further partitioned by the BT tree. Therefore, the QT leaf node isalso the root node for the BT and it has a BT depth of 0. When the BTdepth reaches MaxBTDepth (i.e., 4), no further splitting is considered.When the BT node has a width equal to MinBTSize (i.e., 4), no furtherhorizontal splitting is considered. Similarly, when the BT node has aheight equal to MinBTSize, no further vertical splitting is considered.The leaf nodes of the BT are further processed by prediction andtransform processing without any further partitioning. For example, themaximum CTU size is 256×256 luma samples such as in JEM.

FIG. 8A illustrates an example of a block partition (8100) by using aQTBT partitioning structure (8200) and FIG. 8B illustrates thecorresponding QTBT structure (8200). The solid lines indicate QT splitsand the dotted lines indicate binary tree BT splits. In each non-leafleaf BT split node, a flag is signaled to indicate a splitting type(i.e., a symmetric horizontal split or a symmetric vertical split). Forexample, in the FIG. 8B example, “0” indicates a symmetric horizontalsplit and “1” indicates a symmetric vertical split. For a QT split,however, a split type flag is not indicated or signaled because the QTsplit splits a non-leaf node both horizontally and vertically to producefour smaller blocks with an equal size.

Referring to FIG. 8B, in the QTBT structure (8200), a root node (8201)is first partitioned by a QT structure into QT nodes (8211)-(8214).Accordingly, as shown in FIG. 8A, a coding tree block (8101) ispartitioned into four equal size blocks (8111)-(8114) by the solidlines.

Referring back to FIG. 8B, the QT nodes (8211) and (8212) are furthersplit by two BT splits, respectively. As mentioned above, a BT splitincludes two splitting types (i.e., a symmetric horizontal split and asymmetric vertical split). The BT split for the non-leaf QT node (8211)is indicated as “1” and thus the non-leaf QT node (8211) can be splitinto two nodes (8221) and (8222) by using a symmetric vertical split.The BT split for the non-leaf QT node (8212) is indicated as “0” andthus the non-leaf QT node (8212) can be split into two nodes (8223) and(8224) by using a symmetric horizontal split. The non-leaf QT node(8213) is further split into four nodes (8225)-(8228) by another QTBTstructure. The node (8214) is not further split and thus a leaf node.Accordingly, as shown in FIG. 8A, the block (8111) is verticallypartitioned into two equal size blocks (8121) and (8122), the block(8112) is horizontally partitioned into two equal size blocks, the block(8113) is partitioned into four equal size blocks, and the block (8114)is not further partitioned.

Referring back to FIG. 8B, in deeper levels, some of the nodes, forexample, the nodes (8221)-(8228) are further split while others are not.For example, the non-leaf BT node (8221) is further split into two leafnodes (8231) and (8232) by a symmetric vertical split while the leafnode (8222) is not further split. Accordingly, as shown in FIG. 8A, theblock (8121) is partitioned into two equal size blocks (8131) and (8132)while the block (8122) is not further partitioned.

After splitting of the QTBT structure (8200) is completed, leaf nodesthat are not further split are CUs that are used for prediction andtransform processing. Thus, a CU, a PU that is associated with the CU,and a TU that is associated with the CU can have a same block size inthe QTBT structure. In addition, in the QTBT structure, a CU can includeCBs in different color components. For example, in a 4:2:0 format, oneCU can include one luma CB and two chroma CBs in a P or B slice.However, in some other embodiments, a CU may include CBs in a singlecomponent. For example, in an I slice, one CU may include one luma CB ortwo chroma CBs. That is to say, the QTBT structure supports an abilityfor luma and chroma to have different partitioning structures.

Currently, for P and B slices, the luma and chroma CTBs in one CTU sharethe same QTBT structure. However, for I slices, the luma CTB ispartitioned into CUs by a QTBT structure and the chroma CTBs arepartitioned into chroma CUs by another QTBT structure. Accordingly, a CUin an I slice includes a coding block of the luma component or codingblocks of two chroma components, and a CU in a P or B slice includescoding blocks of all three color components.

Inter prediction for small blocks is restricted to reduce the memoryaccess of motion compensation, for example in HEVC, such thatbi-prediction is not supported for 4×8 and 8×4 blocks, and interprediction is not supported for 4×4 blocks. In the QTBT as implementedin the JEM-7.0, these restrictions are removed.

For example, in VTM6, the coding tree scheme supports the ability forthe luma and chroma to have separate block tree structures. In anexample, for P and B slices, the luma and chroma CTBs in one CTU have toshare the same coding tree structure. However, for I slices, the lumaand chroma can have separate block tree structures. When separate blocktree mode is applied, luma CTB is partitioned into CUs by one codingtree structure, and the chroma CTBs are partitioned into chroma CUs byanother coding tree structure. This means that a CU in an I slice mayinclude a coding block of the luma component or coding blocks of twochroma components, and a CU in a P or B slice always include codingblocks of all three color components unless the video is monochrome.

IV. Block Partitioning Structure Using Triple-Trees (TT)

In addition to the QTBT structure described above, another splittingstructure called multi-type-tree (MTT) structure can be more flexiblethan the QTBT structure. In MTT, other than QT and BT, horizontal andvertical center-side TTs are introduced, as shown in FIG. 9A and FIG.9B.

FIG. 9A shows an example of vertical center-side TT partitioning. Forexample, a block (910) is vertically split into three sub-blocks(911)-(913) where the sub-block (912) is located in the middle of theblock (910).

FIG. 9B shows an example of horizontal center-side TT partitioning. Forexample, a block (920) is horizontally split into three sub-blocks(921)-(923) where the sub-block (922) is located in the middle of theblock (920).

Similar to a BT split, in a TT split, a flag is signaled to indicate asplitting type (i.e., a symmetric horizontal split or a symmetricvertical split). In an example, “0” indicates a symmetric horizontalsplit and “1” indicates a symmetric vertical split.

One benefit of the TT partitioning is that the TT partitioning can be acomplement to the QT partitioning and the BT partitioning. For example,the TT partitioning is able to capture objects which are located in ablock center while the QT partitioning and the BT partitioning arealways splitting along the block center. Another benefit of the TTpartitioning is that the width and height of the partitions through theTT partitioning are always a power of 2, so that no additionaltransforms are needed.

A MTT structure, including quad-tree, binary-tree, and ternary-treesplitting types, can be referred to as a QTBTTT structure. Similar to aQTBT structure, a QTBTTT structure also supports luma and chroma havingdifferent structures. For example, in an I slice, a QTBTTT structureused to partition a luma CTB can be different from a QTBTTT structureused to partition a chroma CTB. This means that when a separated treestructure is enabled, a CU includes one luma CB or two chroma CBs.However, in a P or B slice, a luma CTB can share the same QTBTTTstructure with a chroma CTB in one CTU. This means that when theseparated tree structure is disabled, a CU includes all three CBs (i.e.,one luma CB and two chroma CBs).

Dual-tree or separate-tree can be used for I-slice, such as in VVC. Thatis, one tree is used for a luma component and the other tree is used fora chroma component. For a B-slice and a P-slice, one single-tree can beshared by both luma and chroma components.

The design of a two-level tree is can be motivated by complexityreduction. In an example, the complexity of traversing a tree is TD,where T denotes the number of split types, and D is the depth of tree.

V. Sub-Block transform (SBT)

Spatially varying transform (SVT) can also be referred to as a sub-blocktransform (SBT). The SBT can be applied to inter prediction residuals.For example, a coding block can be partitioned into sub-blocks, and onlypart of the sub-blocks is treated as a residual block. Zero residual isassumed for the remaining part of the sub-blocks. Therefore, theresidual block is smaller than the coding block, and a transform size inSBT is smaller than the coding block size. For the region which is notcovered by the residual block, no transform processing is performed.

FIGS. 10A-10D show exemplary SBT modes according to some embodiments ofthe disclosure. The SBT modes support different SBT types such as SVT-Hand SVT-V (e.g., vertically or horizontally partitions), sizes, andpositions (e.g., left half, left quarter, right half, right quarter, tophalf, top quarter, bottom half, bottom quarter). The shaded regionslabeled by letter “A” correspond to residual blocks with transforms, andthe other regions can be assumed to be zero residual without transform.

FIGS. 11-15 show exemplary syntax related to SBT methods according tosome embodiments of the disclosure. It can be seen that the SBT methodsrequire one or more overhead bits (e.g., cu_sbt_flag, cu_sbt_quad_flag,cu_sbt_horizontal_flag, cu_sbt_pos_flag, etc.) to be signaled toindicate the SBT type or direction (e.g., horizontal or vertical), size(e.g., half or quarter), and position (e.g., left or right, top orbottom).

Specifically, in FIG. 11, when sps_sbt_enable_flag is equal to 0, itspecifies that SBT for the inter-predicted CU is disabled. Whensps_sbt_enable_flag is equal to 1, it specifies that SBT for theinter-predicted CU is enabled.

In FIG. 12, when slice_max_sbt_size_64_flag is equal to 0, it specifiesthat the maximum CU width and height for allowing SBT is 32. Whenslice_max_sbt_size_64_flag is equal to 1, it specifies that the maximumCU width and height for allowing SBT is 64. That is,maxSbtSize=slice_max_sbt_size_64_flag? 64:32.

In FIG. 13, when cu_sbt_flag[x0][y0] is equal to 1, it specifies thatSBT is used for the current CU. When cu_sbt_flag[x0][y0] is equal to 0,it specifies that SBT is not used for the current CU.

When cu_sbt_flag[x0][y0] is not present, its value is inferred to beequal to 0.

It is noted that when SBT is used, a CU is tiled into two TUs, one TUhas residuals, and the other TU does not have residuals.

When cu_sbt_quad_flag[x0][y0] is equal to 1, it specifies that for thecurrent CU, the SBT includes a TU of ¼ size of the current CU. Whencu_sbt_quad_flag[x0][y0] is equal to 0, it specifies that for thecurrent CU the SBT includes a TU of ½ size of the current CU.

When cu_sbt_quad_flag[x0][y0] is not present, its value is inferred tobe equal to 0.

When cu_sbt_horizontal_flag[x0][y0] is equal to 1, it specifies that thecurrent CU is tiled into 2 TUs by a horizontal split. Whencu_sbt_horizontal_flag[x0][y0] is equal to 0, it specifies that thecurrent CU is tiled into 2 TUs by a vertical split.

When cu_sbt_horizontal_flag[x0][y0] is not present, its value can bederived as follows: if cu_sbt_quad_flag[x0][y0] is equal to 1,cu_sbt_horizontal_flag[x0][y0] is set to be equal to allowSbtHoriQuad;otherwise (cu_sbt_quad_flag[x0][y0] is equal to 0),cu_sbt_horizontal_flag[x0][y0] is set to be equal to allowSbtHoriHalf.

When cu_sbt_pos_flag[x0][y0] is equal to 1, it specifies that thetu_cbf_luma, tu_cbf_cb, and tu_cbf_cr of the first TU in the current CUare not present in the bitstream. When cu_sbt_pos_flag[x0][y0] is equalto 0, it specifies that the tu_cbf_luma, tu_cbf_cb, and tu_cbf_cr of thesecond transform unit in the current CU are not present in thebitstream.

VI. Interaction between CU Partitions and SBT

The present disclosure includes embodiments for improving performance ofCU partitions and SBT.

It can be seen in the above discussion that a partition associated withan SBT mode can overlap or correspond to a partition associated with aCU partitioning structure. For example, an SBT mode can be performed ona CU with a size of 16×16 and the left 16×8 sub-CU may be transformed.This process can be mimicked or achieved by a further CU split, afterwhich the residual coding can be performed on the left 16×8 sub-CU whilethe right 16×8 sub-CU has no residual.

This disclosure includes methods for disallowing an SBT mode for acurrent block if a partition associated with the SBT mode can beachieved by a partition associated with a CU partitioning structure. Insome embodiments, when one or more SBT modes (e.g., a vertical SBT mode,a horizontal SBT mode, a half SBT mode, and/or a quarter SBT mode) arenot allowed, the related syntax may be inferred without signaling.

The following embodiments are described using a block width of thecurrent block and a vertical SBT mode as an example. The embodiments canbe extended to the case of a block height and a horizontal SBT mode. Inthe following embodiments, a half SBT mode refers to an SBT mode thatpartitions one CU into two equal-size sub-TUs, and a quarter SBT moderefers to an SBT mode that partitions one CU into a sub-TU withone-quarter size and a sub-TU with three-quarter size. The sub-TU withone-quarter size may have residuals and the sub-TU with three-quartersize may not have any residual. In addition, the term block may beinterpreted as a PB, a coding block, or a CU.

In one embodiment, the vertical SBT mode is not allowed for a blockhaving a width that is greater than a threshold when a partitioningdepth associated with the block is less than a maximum allowedpartitioning depth associated with the block. In this case, thepartition associated with the vertical SBT mode can be mimicked orachieved by one vertical partition, such as vertical BT or TT splits. Inone example, the threshold is 8 for the half SBT mode if BT is allowedfor the block. In another example, the threshold is 16 for the quarterSBT mode if TT is allowed for the block.

In one embodiment, when SBT mode partitioning can lead to small chromablocks, such as 4×2, 2×4, and 2×2 chroma blocks, the SBT mode is notallowed.

In one embodiment, if only one SBT direction is allowed, a flagindicating the other SBT direction is not signaled but can be inferred.For example, cu_sbt_horizontal_flag is not signaled but can be inferred.

In one embodiment, if only some SBT sizes are allowed, a flag indicatingan SBT mode having a minimum allowed SBT size that is larger than thatof another SBT mode is not signaled but can be inferred. For example, ifboth of the half and quarter SBT modes are not allowed, cu_sbt_quad_flagis not signaled but can be inferred.

In one embodiment, if no SBT mode is allowed, a flag (e.g., cu_sbt_flag)indicating whether an SBT mode is performed on the block is not signaledbut can be inferred.

In one embodiment, when a CU partition associated with a block can alignwith a sub-TU partition of a parent CU of the block by using an SBTmode, a transform type for the block can be determined based on the SBTmode. For example, if the block is generated based on a vertical BTsplit that partitions the parent CU into a left sub-CU (i.e., the block)and a right sub-CU, and the parent CU can be also partitioned using avertical half SBT mode into a left sub-TU and a right sub-TU, then theblock can be considered to be aligned with the left sub-TU, and thetransform type of the block can be determined based on a transform typefor the left sub-TU. For example, the transform type of the block can befound in a look-up transform table used for SBT mode based on a size ofthe block. In addition, in an example, a flag or index can be signaledto indicate whether the transform type of the block is DCT-2 or the sameas the transform type used for the sub-TU partition of the parent CU.

According to aspects of the disclosure, context models (e.g., twocontext models) can be used for entropy coding the SBT mode informationwhen related SBT modes are allowed.

In one embodiment, the context model selection for the flag cu_sbt_flagmay be based on whether the partitioning depth (e.g., current BTdepth+1) associated with the block is less than the maximum allowedpartitioning depth (e.g., maximum allowed BT/TT depth).

In one embodiment, the context model selection for the flagcu_sbt_quad_flag may be based on the flag cu_sbt_horizontal_flag and theblock width/height. For example, two context models for the flagcu_sbt_quad_flag may be used based on whether the following condition istrue:(cu_sbt_horizontal_flag&&height>=16)∥(!cu_sbt_horizontal_flag&&width>=16).When the condition is true, one context model is selected. When thecondition is false, the other context mode is selected.

According to aspects of the disclosure, SBT can be disallowed, forexample according to one or more of the above-described conditions, forcertain types of CUs. In one embodiment, diallowing SBT according to oneor more of the above-described conditions can only be applied to anon-merge inter block.

VII. Flowchart

FIG. 16 shows a flow chart outlining an exemplary process (1600)according to an embodiment of the disclosure. In various embodiments,the process (1600) is executed by processing circuitry, such as theprocessing circuitry in the terminal devices (210), (220), (230) and(240), the processing circuitry that performs functions of the videoencoder (303), the processing circuitry that performs functions of thevideo decoder (310), the processing circuitry that performs functions ofthe video decoder (410), the processing circuitry that performsfunctions of the intra prediction module (452), the processing circuitrythat performs functions of the video encoder (503), the processingcircuitry that performs functions of the predictor (535), the processingcircuitry that performs functions of the intra encoder (622), theprocessing circuitry that performs functions of the intra decoder (772),and the like. In some embodiments, the process (1600) is implemented insoftware instructions, thus when the processing circuitry executes thesoftware instructions, the processing circuitry performs the process(1600).

The process (1600) may generally start at step (S1610), where theprocess (1600) decodes prediction information of a current block of acoding unit tree in a coded bit stream. The prediction informationindicates at least one allowed block partitioning structure for thecurrent block. Then, the process (1600) proceeds to step (S1620).

At step (S1620), the process (1600) determines a sub-block transform(SBT) mode for the current block based on the prediction informationindicating that SBT is used for the current block. A partition of thecurrent block based on the SBT mode is different from a partition of thecurrent block based on the at least one allowed block partitioningstructure. Then, the process (1600) proceeds to step (S1630).

At step (S1630), the process (1600) reconstructs the current block basedon the SBT mode. Then, the process (1600) terminates.

In an embodiment, the process (1600) determines the SBT mode for thecurrent block when (i) one of a width and a height of the current blockis greater than a first threshold and (ii) a partitioning depthassociated with the current block is less than a maximum allowedpartitioning depth of the coding unit tree. In an example, the SBT modeis determined not to be a vertical SBT mode when the width of thecurrent block is greater than the first threshold. In an example, theSBT mode is determined not to be a horizontal SBT mode when the heightof the current block is greater than the first threshold. In an example,the SBT mode is determined not to be a half SBT mode when (i) the firstthreshold is 8 and (ii) the at least one allowed partitioning structureincludes a binary tree partitioning structure. In an example, the SBTmode is determined not to be a quarter SBT mode when (i) the firstthreshold is 16 and (ii) the at least one allowed partitioning structureincludes a triple tree partitioning structure.

In an embodiment, the process (1600) determines the SBT mode for thecurrent block when (i) the current block is a chroma block and (ii)sizes of a plurality of sub-blocks of the current block are greater thana second threshold. The current block can be partitioned into theplurality of sub-blocks based on the SBT mode.

In an embodiment, a flag indicating a direction of the SBT mode is notincluded in the prediction information.

In an embodiment, a flag indicating a size of the SBT mode is notincluded in the prediction information.

In an embodiment, a flag indicating whether SBT is used for the currentblock is not included in the prediction information.

In an embodiment, the process (1600) determines a context model for aflag based on a partitioning depth associated with the current block.The flag indicates whether SBT is used for the current block.

In an embodiment, the process (1600) determines a context model for aflag based on at least one of a direction of the SBT mode, the width ofthe current block, and the height of the current block. The flagindicates a size of the SBT mode.

In an embodiment, the prediction information indicates that the currentblock is a non-merge inter block.

VIII. Computer System

The presented methods may be used separately or combined in any order.Further, each of the embodiments, encoder, and decoder may beimplemented by processing circuitry (e.g., one or more processors or oneor more integrated circuits). In one example, the one or more processorsexecute a program that is stored in a non-transitory computer-readablemedium.

The techniques described above, can be implemented as computer softwareusing computer-readable instructions and physically stored in one ormore computer-readable media. For example, FIG. 17 shows a computersystem (1700) suitable for implementing certain embodiments of thedisclosed subject matter.

The computer software can be coded using any suitable machine code orcomputer language, that may be subject to assembly, compilation,linking, or like mechanisms to create code comprising instructions thatcan be executed directly, or through interpretation, micro-codeexecution, and the like, by one or more computer central processingunits (CPUs), Graphics Processing Units (GPUs), and the like.

The instructions can be executed on various types of computers orcomponents thereof, including, for example, personal computers, tabletcomputers, servers, smartphones, gaming devices, internet of thingsdevices, and the like.

The components shown in FIG. 17 for computer system (1700) are exemplaryin nature and are not intended to suggest any limitation as to the scopeof use or functionality of the computer software implementingembodiments of the present disclosure. Neither should the configurationof components be interpreted as having any dependency or requirementrelating to any one or combination of components illustrated in theexemplary embodiment of a computer system (1700).

Computer system (1700) may include certain human interface inputdevices. Such a human interface input device may be responsive to inputby one or more human users through, for example, tactile input (such as:keystrokes, swipes, data glove movements), audio input (such as: voice,clapping), visual input (such as: gestures), olfactory input (notdepicted). The human interface devices can also be used to capturecertain media not necessarily directly related to conscious input by ahuman, such as audio (such as: speech, music, ambient sound), images(such as: scanned images, photographic images obtain from a still imagecamera), video (such as two-dimensional video, three-dimensional videoincluding stereoscopic video).

Input human interface devices may include one or more of (only one ofeach depicted): keyboard (1701), mouse (1702), trackpad (1703), touchscreen (1710), data-glove (not shown), joystick (1705), microphone(1706), scanner (1707), camera (1708).

Computer system (1700) may also include certain human interface outputdevices. Such human interface output devices may be stimulating thesenses of one or more human users through, for example, tactile output,sound, light, and smell/taste. Such human interface output devices mayinclude tactile output devices (for example tactile feedback by thetouch-screen (1710), data-glove (not shown), or joystick (1705), butthere can also be tactile feedback devices that do not serve as inputdevices), audio output devices (such as: speakers (1709), headphones(not depicted)), visual output devices (such as screens (1710) toinclude CRT screens, LCD screens, plasma screens, OLED screens, eachwith or without touch-screen input capability, each with or withouttactile feedback capability—some of which may be capable to output twodimensional visual output or more than three dimensional output throughmeans such as stereographic output; virtual-reality glasses (notdepicted), holographic displays and smoke tanks (not depicted)), andprinters (not depicted). These visual output devices (such as screens(1710)) can be connected to a system bus (1748) through a graphicsadapter (1750).

Computer system (1700) can also include human accessible storage devicesand their associated media such as optical media including CD/DVD ROM/RW(1720) with CD/DVD or the like media (1721), thumb-drive (1722),removable hard drive or solid state drive (1723), legacy magnetic mediasuch as tape and floppy disc (not depicted), specialized ROM/ASIC/PLDbased devices such as security dongles (not depicted), and the like.

Those skilled in the art should also understand that term “computerreadable media” as used in connection with the presently disclosedsubject matter does not encompass transmission media, carrier waves, orother transitory signals.

Computer system (1700) can also include a network interface (1754) toone or more communication networks (1755). The one or more communicationnetworks (1755) can for example be wireless, wireline, optical. The oneor more communication networks (1755) can further be local, wide-area,metropolitan, vehicular and industrial, real-time, delay-tolerant, andso on. Examples of the one or more communication networks (1755) includelocal area networks such as Ethernet, wireless LANs, cellular networksto include GSM, 3G, 4G, 5G, LTE and the like, TV wireline or wirelesswide area digital networks to include cable TV, satellite TV, andterrestrial broadcast TV, vehicular and industrial to include CANBus,and so forth. Certain networks commonly require external networkinterface adapters that attached to certain general purpose data portsor peripheral buses (1749) (such as, for example USB ports of thecomputer system (1700)); others are commonly integrated into the core ofthe computer system (1700) by attachment to a system bus as describedbelow (for example Ethernet interface into a PC computer system orcellular network interface into a smartphone computer system). Using anyof these networks, computer system (1700) can communicate with otherentities. Such communication can be uni-directional, receive only (forexample, broadcast TV), uni-directional send-only (for example CANbus tocertain CANbus devices), or bi-directional, for example to othercomputer systems using local or wide area digital networks. Certainprotocols and protocol stacks can be used on each of those networks andnetwork interfaces as described above.

Aforementioned human interface devices, human-accessible storagedevices, and network interfaces can be attached to a core (1740) of thecomputer system (1700).

The core (1740) can include one or more Central Processing Units (CPU)(1741), Graphics Processing Units (GPU) (1742), specialized programmableprocessing units in the form of Field Programmable Gate Areas (FPGA)(1743), hardware accelerators for certain tasks (1744), and so forth.These devices, along with Read-only memory (ROM) (1745), Random-accessmemory (1746), internal mass storage such as internal non-useraccessible hard drives, SSDs, and the like (1747), may be connectedthrough the system bus (1748). In some computer systems, the system bus(1748) can be accessible in the form of one or more physical plugs toenable extensions by additional CPUs, GPU, and the like. The peripheraldevices can be attached either directly to the core's system bus (1748),or through a peripheral bus (1749). Architectures for a peripheral businclude PCI, USB, and the like.

CPUs (1741), GPUs (1742), FPGAs (1743), and accelerators (1744) canexecute certain instructions that, in combination, can make up theaforementioned computer code. That computer code can be stored in ROM(1745) or RAM (1746). Transitional data can be also be stored in RAM(1746), whereas permanent data can be stored for example, in theinternal mass storage (1747). Fast storage and retrieve to any of thememory devices can be enabled through the use of cache memory, that canbe closely associated with one or more CPU (1741), GPU (1742), massstorage (1747), ROM (1745), RAM (1746), and the like.

The computer readable media can have computer code thereon forperforming various computer-implemented operations. The media andcomputer code can be those specially designed and constructed for thepurposes of the present disclosure, or they can be of the kind wellknown and available to those having skill in the computer software arts.

As an example and not by way of limitation, the computer system havingarchitecture (1700), and specifically the core (1740) can providefunctionality as a result of processor(s) (including CPUs, GPUs, FPGA,accelerators, and the like) executing software embodied in one or moretangible, computer-readable media. Such computer-readable media can bemedia associated with user-accessible mass storage as introduced above,as well as certain storage of the core (1740) that are of non-transitorynature, such as core-internal mass storage (1747) or ROM (1745). Thesoftware implementing various embodiments of the present disclosure canbe stored in such devices and executed by core (1740). Acomputer-readable medium can include one or more memory devices orchips, according to particular needs. The software can cause the core(1740) and specifically the processors therein (including CPU, GPU,FPGA, and the like) to execute particular processes or particular partsof particular processes described herein, including defining datastructures stored in RAM (1746) and modifying such data structuresaccording to the processes defined by the software. In addition or as analternative, the computer system can provide functionality as a resultof logic hardwired or otherwise embodied in a circuit (for example:accelerator (1744)), which can operate in place of or together withsoftware to execute particular processes or particular parts ofparticular processes described herein. Reference to software canencompass logic, and vice versa, where appropriate. Reference to acomputer-readable media can encompass a circuit (such as an integratedcircuit (IC)) storing software for execution, a circuit embodying logicfor execution, or both, where appropriate. The present disclosureencompasses any suitable combination of hardware and software.

While this disclosure has described several exemplary embodiments, thereare alterations, permutations, and various substitute equivalents, whichfall within the scope of the disclosure. It will thus be appreciatedthat those skilled in the art will be able to devise numerous systemsand methods which, although not explicitly shown or described herein,embody the principles of the disclosure and are thus within the spiritand scope thereof.

APPENDIX A: ACRONYMS

-   AMT: Adaptive Multiple Transform-   AMVP: Advanced Motion Vector Prediction-   ASIC: Application-Specific Integrated Circuit-   ATMVP: Alternative/Advanced Temporal Motion Vector Prediction-   BDOF: Bi-directional Optical Flow-   BDPCM (or RDPCM): Residual Difference Pulse Coded Modulation-   BIO: Bi-directional Optical Flow-   BMS: Benchmark Set-   BT: Binary Tree-   BV: Block Vector-   CANBus: Controller Area Network Bus-   CB: Coding Block-   CBF: Coded Block Flag-   CCLM: Cross-Component Linear Mode/Model-   CD: Compact Disc-   CPR: Current Picture Referencing-   CPU: Central Processing Unit-   CRT: Cathode Ray Tube-   CTB: Coding Tree Block-   CTU: Coding Tree Unit-   CU: Coding Unit-   DM: Derived Mode-   DPB: Decoder Picture Buffer-   DVD: Digital Video Disc-   EMT: Enhanced Multiple Transform-   FPGA: Field Programmable Gate Areas-   GOP: Group of Picture-   GPU: Graphics Processing Unit-   GSM: Global System for Mobile communications-   HDR: High Dynamic Range-   HEVC: High Efficiency Video Coding-   HRD: Hypothetical Reference Decoder-   IBC: Intra Block Copy-   IC: Integrated Circuit-   IDT: Identify Transform-   ISP: Intra Sub-Partitions-   JEM: Joint Exploration Model-   JVET: Joint Video Exploration Team-   LAN: Local Area Network-   LCD: Liquid-Crystal Display-   LFNST: Low Frequency Non-Separable Transform, or Low Frequency    Non-Separable Secondary Transform-   LTE: Long-Term Evolution-   L_CCLM: Left-Cross-Component Linear Mode/Model-   LT_CCLM: Left and Top Cross-Component Linear Mode/Model-   MIP: Matrix based Intra Prediction-   MPM: Most Probable Mode-   MRLP (or MRL): Multiple Reference Line Prediction-   MTS: Multiple Transform Selection-   MV: Motion Vector-   NSST: Non-Separable Secondary Transform-   OLED: Organic Light-Emitting Diode-   PBs: Prediction Blocks-   PCI: Peripheral Component Interconnect-   PDPC: Position Dependent Prediction Combination-   PLD: Programmable Logic Device-   PPR: Parallel-Processable Region-   PPS: Picture Parameter Set-   PU: Prediction Unit-   QT: Quad-Tree-   RAM: Random Access Memory-   ROM: Read-Only Memory-   RST: Reduced-Size Transform-   SBT: Sub-block Transform-   SCC: Screen Content Coding-   SCIPU: Small Chroma Intra Prediction Unit-   SDR: Standard Dynamic Range-   SEI: Supplementary Enhancement Information-   SNR: Signal Noise Ratio-   SPS: Sequence Parameter Set-   SSD: Solid-state Drive-   SVT: Spatially Varying Transform-   TSM: Transform Skip Mode-   TT: Ternary Tree-   TU: Transform Unit-   T_CCLM: Top Cross-Component Linear Mode/Model-   USB: Universal Serial Bus-   VPDU: Visual Process Data Unit-   VPS: Video Parameter Set-   VUI: Video Usability Information-   VVC: Versatile Video Coding-   WAIP: Wide-Angle Intra Prediction

What is claimed is:
 1. A method for video coding in a decoder,comprising: decoding prediction information of a current block of acoding unit tree in a coded bit stream, the prediction informationindicating at least one allowed block partitioning structure for thecurrent block; determining a sub-block transform (SBT) mode for thecurrent block based on the prediction information indicating that SBT isused for the current block, a partition of the current block based onthe SBT mode being different from a partition of the current block basedon the at least one allowed block partitioning structure; andreconstructing the current block based on the SBT mode.
 2. The method ofclaim 1, wherein the determining the SBT mode for the current blockincludes: determining the SBT mode for the current block based on (i)one of a width and a height of the current block being greater than afirst threshold and (ii) a partitioning depth associated with thecurrent block being less than a maximum allowed partitioning depth ofthe coding unit tree.
 3. The method of claim 2, wherein the determiningthe SBT mode for the current block includes at least one of: determiningthat the SBT mode is not a vertical SBT mode based on the width of thecurrent block being greater than the first threshold; and determiningthat the SBT mode is not a horizontal SBT mode based on the height ofthe current block being greater than the first threshold.
 4. The methodof claim 2, wherein the determining the SBT mode for the current blockincludes at least one of: determining that the SBT mode is not a halfSBT mode based on (i) the first threshold being 8 and (ii) the at leastone allowed partitioning structure including a binary tree partitioningstructure; and determining that the SBT mode is not a quarter SBT modebased on (i) the first threshold being 16 and (ii) the at least oneallowed partitioning structure including a triple tree partitioningstructure.
 5. The method of claim 1, wherein the determining the SBTmode for the current block includes: determining the SBT mode for thecurrent block based on (i) the current block being a chroma block and(ii) sizes of a plurality of sub-blocks of the current block beinggreater than a second threshold, the current block being partitionedinto the plurality of sub-blocks based on the SBT mode.
 6. The method ofclaim 1, wherein a flag indicating a direction of the SBT mode is notincluded in the prediction information.
 7. The method of claim 1,wherein a flag indicating a size of the SBT mode is not included in theprediction information.
 8. The method of claim 1, wherein a flagindicating whether SBT is used for the current block is not included inthe prediction information.
 9. The method of claim 1, furthercomprising: determining a context model for a flag based on apartitioning depth associated with the current block, the flagindicating whether SBT is used for the current block.
 10. The method ofclaim 2, further comprising: determining a context model for a flagbased on at least one of a direction of the SBT mode, the width of thecurrent block, and the height of the current block, the flag indicatinga size of the SBT mode.
 11. The method of claim 1, wherein theprediction information indicates that the current block is a non-mergeinter block.
 12. An apparatus, comprising processing circuitryconfigured to: decode prediction information of a current block of acoding unit tree in a coded bit stream, the prediction informationindicating at least one allowed block partitioning structure for thecurrent block; determine a sub-block transform (SBT) mode for thecurrent block based on the prediction information indicating that SBT isused for the current block, a partition of the current block based onthe SBT mode being different from a partition of the current block basedon the at least one allowed block partitioning structure; andreconstruct the current block based on the SBT mode.
 13. The apparatusof claim 11, wherein the processing circuitry is further configured to:determine the SBT mode for the current block based on (i) one of a widthand a height of the current block being greater than a first thresholdand (ii) a partitioning depth associated with the current block beingless than a maximum allowed partitioning depth of the coding unit tree.14. The apparatus of claim 13, wherein the processing circuitry isfurther configured to perform at least one of: determining that the SBTmode is not a vertical SBT mode based on the width of the current blockbeing greater than the first threshold; and determining that the SBTmode is not a horizontal SBT mode based on the height of the currentblock being greater than the first threshold.
 15. The apparatus of claim13, wherein the processing circuitry is further configured to perform atleast one of: determining that the SBT mode is not a half SBT mode basedon (i) the first threshold being 8 and (ii) the at least one allowedpartitioning structure including a binary tree partitioning structure;and determining that the SBT mode is not a quarter SBT mode based on (i)the first threshold being 16 and (ii) the at least one allowedpartitioning structure including a triple tree partitioning structure.16. The apparatus of claim 11, wherein the processing circuitry isfurther configured to: determine the SBT mode for the current blockbased on (i) the current block being a chroma block and (ii) sizes of aplurality of sub-blocks of the current block being greater than a secondthreshold, the current block being partitioned into the plurality ofsub-blocks based on the SBT mode.
 17. The apparatus of claim 11, whereina flag indicating a direction of the SBT mode is not included in theprediction information.
 18. The apparatus of claim 11, wherein a flagindicating a size of the SBT mode is not included in the predictioninformation.
 19. The apparatus of claim 1, wherein a flag indicatingwhether SBT is used for the current block is not included in theprediction information.
 20. A non-transitory computer-readable storagemedium storing a program executable by at least one processor toperform: decoding prediction information of a current block of a codingunit tree in a coded bit stream, the prediction information indicatingat least one allowed block partitioning structure for the current block;determining a sub-block transform (SBT) mode for the current block basedon the prediction information indicating that SBT is used for thecurrent block, a partition of the current block based on the SBT modebeing different from a partition of the current block based on the atleast one allowed block partitioning structure; and reconstructing thecurrent block based on the SBT mode.